A group of products produced in large quantities by petroleum refineries is the middle distillate products, including materials such as diesel fuel, jet fuel and home heating oil. Some of these products may be relatively higher grade fuel products than others and may command a premium price as fuels products. For example, certain jet fuels of high quality may be sold as premium products with a correspondingly higher price. With a progressively increasing demand for these fuel products, there is a need for increasing the size of the fuel products pool and to this end, various synthetic and semi-synthetic processes for producing both gasoline and middle distillate products have been devised.
One process which has commended itself is the so-called MOGDL process in which light olefins, especially olefinic streams from a catalytic cracking unit, are oligomerized to products in the gasoline boiling range and above by the use of an intermediate pore size zeolite oligomerization catalyst. Processes of this type are disclosed, for example, in U.S. Patents Nos. 3,760,024, 4,211,640, 4,277,992, 4,513,156 and 4,476,330. Reference is made to these patents for descriptions of such processes. The products from these processes tend, however, to be aromatic in character under the conditions generally employed. For this reason, the middle distillate products are generally unsatisfactory for use as jet or diesel fuels which require the aromatic content to be kept at a low level.
Another semi-synthetic process which has been proposed converts low quality paraffin streams to higher value products, generally of an aromatic character, especially high octane gasoline. These processes are generally to be characterized as aromatization processes which convert the paraffins present in the feed to aromatic products, usually in the gasoline boiling range. The aromatic character of the products renders them generally unsatisfactory for use as jet or diesel fuels, although they are good blending components for the gasoline pool. Examples of such processes are to be found in U.S. Pat. No. 3,729,409 which discloses a process for upgrading a reformate feedstock containing paraffins by contacting the feed with a zeolite ZSM-5 catalyst, optionally containing a metal component, at relatively high conversion temperatures, typically from 500.degree. to 1000.degree. F. Similarly, U.S. Pat. No. 3,951,781 discloses a process for upgrading a straight run naphtha in combination with a solvent extracted resin and paraffinic material using a ZSM-5 catalyst, optionally with a metal component. Other aromatization processes are disclosed in U.S. Patents Nos. 4,590,323, 4,304,657 and the article by Garwood and Chen, ACS 125( 1), 84(1980): "Octane Boosting Potential of Catalystic Processing of Reformate Over Shape Selective Zeolites" discloses the conversion of n-octane to benzene over a ZSM-5 catalyst with the production of only trace amounts of C.sub.2 - in the product.
Given the emphasis in these proposals on the production of aromatic gasolines, it is surprising to find that paraffinic products of higher boiling point may be produced by appropriate selection of reaction conditions.